1. Field of the Invention
The present invention relates generally to a noise erasing apparatus for a stimulable phosphor sheet, and more particularly to an improved power supply for supplying power to a light source for emitting light to conduct a second erasing for stimulable phosphor shot.
2. Discussion of Background
As is well known in the art, a photographic method using a silver salt, such as radiography, in which an X-ray film having an emulsion layer including a silver salt is used in combination with an intensifying screen has generally been employed to obtain a radiation image. Recently, due to various problems, such as a shortage of silver resources, a method of obtaining a radiation image without using a silver salt has been desired.
One radiation image recording technique is disclosed in U.S. Pat. No. 3,859,527, entitled, APPARATUS AND METHOD FOR PRODUCING IMAGES CORRESPONDING TO PATTERNS OF HIGH ENERGY RADIATION, issued to Lucky. This patent discloses a method of radiation image recording which employs a radiation image storage panel including a stimulable phosphor which emits light when stimulated by an electromagnetic wave selected from visible light and infrared rays after exposure to radiation such as X-rays, .alpha.-rays, .beta.-rays, high energy neutrons, electron beams, vacuum ultra-violet rays, ultra-violet rays and other similar electromagnetic waves or corpuscular beams.
Radiation image systems successfully using the stimulable phosphor in radiography are described in detail in U.S. Pat. No. 4,236,078, entitled, METHOD AND APPARATUS FOR RECORDING AND REPRODUCING A RADIATION IMAGE, issued to Kotera, et al.
In the radiation image recording and reproducing method described above, the final visible image may be reproduced in the form of a hard copy or may be displayed on a cathode ray tube. The stimulable phosphor sheet used in this method may take various forms such as panel, drum, or the like, which are herein generally referred to as imaging plates (IP). For economical reasons of economy, it is desirable that the IP be used repeatedly in many separate radiographic operations, such as 1,000 operations.
Two problems have been posed in reusing the stimulable phosphor sheet in the course of the radiation image recording and reproducing systems and processes as described above.
One problem has been that a part of the previously stored radiation image remains in the stimulable phosphor sheet after use and disadvantageously causes noise to occur in the visible image reproduced from the stimulable phosphor sheet when it is reused. Theoretically, the radiation energy of the radiation image stored in the stimulable phosphor sheet should disappear when the sheet is scanned with a stimulating ray of a sufficient intensity to release sequentially the radiation energy stored as light emission. However, in actuality, the stored radiation energy cannot be completely eliminated only by the scanning of the stimulating ray. Therefore, a first erasing in which the radiation image previously stored in the stimulable phosphor is to be erased at high illumination for a long length of time by use of a large-scale device can be carried out before the stimulable phosphor sheet is fed in performing radiography.
Another problem has been that a stimulable phosphor contains a trace of radioactive isotopes such as .sup.226 Ra and .sup.40 K, which emit radiation and cause the stimulable phosphor sheet to store the emitted radiation energy even when the sheet is not being used in radiography. These traces of radioactive isotopes also constitute a cause of the noise developed in the reproduced visible radiation image. Further, a stimulable phosphor sheet is also affected by environmental radiations such as cosmic rays and X-rays emitted from other X-ray sources and also stores the such energy. These types of radiation energy also cause noise to appear in the visible radiation image reproduced therefrom. In order to erase the effects of such obstructive radiation energy, a second erasing can be conducted in a short length of time immediately before the next radiography operation by a small-scale erasing device positioned in the radiographic systems. A device and method of this kind is known from U.S. Pat. Nos. 4,438,333, entitled NOISE ERASING APPARATUS FOR STIMULABLE PHOSPHOR SHEET, issued to Teraoka et al, and 4,439,682, entitled NOISE ERASING METHOD IN A RADIATION IMAGE RECORDING AND REPRODUCING METHOD, issued to Matsumoto et al.
An optimized second erasing device, i.e., an illuminant must meet at least the following three requirements:
1. It must have an appropriate illumination of light from light source to complete the erasing operation quickly.
2. It must not emit ultra-violet rays which constitute a cause of noise developed during the second erasing operation.
3. It must not increase the temperature due to exposure by light in order to protect the supporting plate for guiding and supporting and guiding the stimulable phosphor sheet.
The illuminant may for example be a tungsten-filament, fluorescent, sodium, xenon or iodine lamp, or the like.
Regarding the above illuminant, it is well known that the illuminance depends on the luminance of the lamp used in such a device, and the cause of noise as described previously and temperature radiation increases with decreasing of the color temperature in the illuminant. Further, the luminance of lamp and the color temperature of the lamp are determined in accordance with the load power to the lamp.
To maintain a substantially constant load power of the lamp is required so as to regulate the luminance and color temperature of the lamp in accordance with the above requirements, it being understood that in the present application, the load is a lamp. The load power (P.sub.l) is given as follows: EQU P.sub.l (Load power)=E.sub.l (Input voltage supplied to the filament in the lamp).times.I.sub.l (Filament current).
In the prior art, regulation of the input power applied to the lamp in the erasing device is performed by detecting the line voltage in the line connected between the lamp and the power regulator which regulates the input power applied to the lamp instead of the actual input voltage. However, the line voltage is different from the actual input voltage due to cable resistance in the line, therefore, which makes it difficult to regulate the input power supplied to the lamp with accuracy.